As we know, the temperature is directly proportional to pH. It means low temperature and low pH during the morning and high temperature and high pH during the evening in an aquatic ecosystem/pond/outdoor cemented tanks. But, I observed an inverse relationship between temperature and pH in indoor tanks in wet-lab. What is the reason for the above mentioned?
Temperature plays a significant role on pH measurements. As the temperature rises, molecular vibrations increase which results in the ability of water to ionise and form more hydrogen ions. As a result, the pH will drop. The dissociation of water into hydrogen and hydroxide ion can be represented as:
H2O (l) ⇌ H+ (aq) + OH− (aq)
Every solution will undergo a change in their pH value through changes in temperature. A difference in pH measurements at different temperatures is NOT an error! The new pH level simply tells about the true pH for that solution at that specific temperature.
It is clearly evident from the table that the pH of water at 0°C is 7.47, but the same water at 100°C will have a pH of 6.14*.
Your text suggests that when the temperature rises, the pH increases (evening) and vice versa in the morning. To my knowledge, pH decreases with increase in temperature. With an increase in temperature more energy is there for molecular vibrations and consequently water molecules fall apart more easily. Therefore, the equilibrium concentration of H+ rises, causing the pH of water get decreased with rise in temperature.
Unless I do not understand your question.
Regards
Your question is teleological. There is no reason, only a cause. You can determine cause and effect, but not why. This is because models can only reject a null hypothesis, not tell you the reason why. But there could be a chemical reason, as suggested by Bachir and Ahmed.
Thank you so much sir for sharing your knowledge and experience. As per my knowledge, an increase in temperature of water is responsible for higher metabolic rate of aquatic animals which results in an increase of pH of water. I had observed similar results i.e. low temperature and pH in morning and vice-versa in evening in indoor tanks, outdoor tanks and ponds (freshwater) earlier but I observed inverse relationship between temperature and pH in indoor tanks (inland saline water) recently @Dr Ahmed B. @Dr. Bachir A
Photosynthesis is an important driver here. The plants (algae and macrophytes) take up the carbon dioxide during photosynthesis, which peaks in the afternoon (sunlight + temperature). In more eutrophic systems, the plants take up more carbon dioxide. Carbon dioxide reaeration from the atmosphere eventually replaces the lost CO2 after photosynthesis stops. The carbon dioxide dissociates into carbonic acid, dropping the pH. If you're inside, then you don't have so much photosynthesis, so the temperature-pH effect can have an over-riding effect.
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